It is known that impurities can be removed from different media by adsorption treatment in a fixed bed of solid adsorbents (U.S. Pat. No. 5,360,547, U.S. Pat. No. 5,220,099, U.S. Pat. No. 4,677,231). Use of sulphonic acid containing activated carbon in the removal of organic cations from polar liquids is, furthermore, disclosed in U.S. Pat. No. 4,968,433.